If you are thinking about ordering your glasses or spectacles online, then obviously a concern is whether they will match your glasses prescription and you will have good eyesight after receiving your glasses. Just because it is cheap to buy glasses online does not mean that they should not perfectly match your glasses prescription and your eyesight should be exactly corrected.
Usually the definition of perfect eyesight is termed as twenty twenty vision - but what does this actually mean ?
If you believe that you have perfect or "20/20" vision when wearing your glasses, would this mean:
(a) Vision is accurate enough to pass a driving test i.e. you can see and decipher a standard number plate on a vehicle from twenty metres. (b) Good enough vision to meet any required occupational standard such as that needed to become a Police Officer or an Airline Pilot. (c) been a couch potato too much and have absorbed too many Americanisms?
The answer is that with your new glasses you have all three of the above and in particular (c), because in all countries of the world, with the exception of the United States, the standard of perfect visual acuity is expressed as 6/6. Now the mathematically minded reader will immediately notice that the one standard is based on metric measurement and the other on imperial, 20 feet being very very nearly the same as 6 metres. The standard of 6/6 relates to the "Snellen" Test Type, of which everybody, at some time, must have been asked to read the letters, whether in the course of a sight test or simply a routine medical examination. The design of the Snellen test type is based on the assumption that the normal eye can recognise or resolve details that subtend an angle of one minute; or 1.75 mm at 6 metres.
Now some readers will have been aware of this useful piece of information already and others may feel that could happily have passed through life without ever being made aware of it.
So why is that Britain, with its imperialist history has always used the metric system and the United States which once declined to remain a member of the British Empire has always used the imperial measurements.
Perhaps the USA could be persuaded to use 6/6 instead of 20/20. Would this make it eligible to apply to be reinstated as a member of the, somewhat reduced British Empire? Of course this may require some juggling of their standards and definitions - indeed it may even require a new act of congress to be passed so that the "new" country may fall back in line with the "old" country. Another way of looking at it however is that by this one small step, the modern Europe may bring the modern United States kicking and screaming into the twenty first century to use a metric system that has long been adopted as a worldwide standard and in then makes a lot more arithmetic sense.
This could be a significant contribution from the world of optics to world harmony !
Even better, we could insist that all UK members who currently say twenty twenty mow move to the metric definition of 6/6.
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If you are looking for twenty twenty vision and considering buying glasses online, we strongly recommend that you read our comprehensiveguide to buying glasses online first
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